The present invention generally relates to a system for fixing the relative positions of one or more bone segments by the use of bone plates and screws, and more specifically to such a system which prevents screw back-out without resort to secondary locking devices.
The use of bone plate and bone screw fixation systems for treating injuries to bones is well established. In most instances, a bone plate is engaged to a bone with the plate over and surrounding the bone injury area. The bone plate is affixed to the bone by bone screws or other similar fasteners inserted through holes in the bone plate and into the bone itself. The screws are tightened so that the bone plate holds the bone to be treated in place in order to insure proper healing. Early fixation devices tended to be applicable only to long bone injuries with some limited uses for lower lumbar spinal injuries and disorders. The use of plate/screw fixation systems expanded, however, to include uses for more spinal injuries and fusion of vertebrae including fixation devices for treating cervical vertebrae injuries. While these systems are applicable to spinal injuries, they tend to encounter a variety of problems which lead to less than optimal results. These problems include, amongst others, xe2x80x9cbackout.xe2x80x9d
Backout is the exhibited tendency of bone screws, which affix the bone plate to the bone(s), to loosen with respect to both the plate and bone resulting in poor fixation, fusion and ultimately, healing. Essentially, this loosening of the bone screw causes the screw to work itself out of the bone into which it is implanted. This results in the bone plate being poorly fixed in place thus becoming devoid of its fixation capabilities. Usually, backout is caused by the stress of bodily movement. While such loosening can be benign if limited in scope, it more often leads to complications such as complete failure of the fixation device or incomplete bone fusion. Backout is particularly prevalent in areas of high bodily stress, such as the spine.
To alleviate backout and its associated problems, current systems utilize secondary locking screws, locking collars or other secondary locking devices that hold the bone screws in place after deployment within the bone. In most systems, the bone screw is affixed into the bone through an opening in a bone plate. A locking device is then inserted into the bone screw. The locking device engages the head of the bone screw and is tightened which results in the bone screw being fixed in place within the bone, thus preventing backout.
While a locking screw or collar can alleviate backout, successful use of such locking device systems in the anterior cervical spine is particularly difficult because limited operating space is available due to anatomic constraints. Systems using multiple types of screws or collars to hold the bone screw in place are difficult to deploy within the confines of the small operating area of the cervical spine. Furthermore, due to the small operating area, the surgeon implanting the device has great difficulty determining if the device is properly deployed. Any instrumentation implanted in the region must be minimally intrusive, yet have adequate strength to withstand the biomechanical loads to which it will be subjected. Thus, while current systems can help reduce instances of backout, their complex nature makes proper deployment very difficult and increases the chance of surgical error.
Current treatment methods also call for instrumentation which is able to provide both rigid fixation and semi-rigid, or xe2x80x9cdynamized,xe2x80x9d fixation that allows the implant to accommodate graft settling. Backout, however, limits the use of such dynamized fixation because the locking devices do not accommodate for graft settling.
Prior art systems, while addressing some of the requirements for successful anterior cervical spinal instrumentation, share several other common deficiencies. Early systems were typically produced using stainless steel components. These stainless steel components interfered with magnetic resonance imaging (MRI) equipment, resulting in poor-quality postoperative imaging of the fixation system. Current systems have changed materials to commercially pure titanium and titanium alloy in order to reduce the MRI noise. However, some noise artifacts remain. Also, the anterior-posterior image obtained via X-ray based methods cannot be used to evaluate graft healing progress, since metallic implant components block X-rays.
Thus, a need exists for an instrumentation system which decreases the surgical complexity of anterior cervical instrumentation and eliminates backout while providing enhanced postoperative imaging possibilities and structural integrity. Reducing the complexity of the instrumentation decreases the chance for surgical error, reduces the time required to implant a fixation device, and reduces the cost of the surgery. Providing enhanced postoperative imaging capability increases the surgeon""s ability to evaluate the healing progress. Accordingly, a system allowing for easy deployment while eliminating backout, retaining structural integrity and improving imaging capabilities is needed in the art.
The bone fixation system of the present invention addresses and overcomes problems found in the prior art. In accordance with one aspect of the invention, a system for anterior fixation of bones of the cervical spine is provided which includes an elongated bone plate sufficient in length to span at least two vertebrae with the plate including one or more holes shaped to accept the head of a mating fastener such as a bone screw.
To eliminate backout, one or more of the holes in the device are formed by creating an undercut within at least a portion of the hole. Preferably, the undercut has a spherical concavity configuration, but other geometries can also perform the required functions of the device and eliminate back-out. Bone screws are provided with a head portion whose geometrical configuration allows for engaging of the head portion with the undercut. As the screw is driven through the plate and into the bone, the head portion of the screw will engage the undercut ultimately xe2x80x9csnappingxe2x80x9d into the undercut which then securely retains the screw and provides sufficient force to prevent postoperative backout.
In accordance with another aspect of the present invention, in the event that a screw does need to be removed from the bone and/or plate or be repositioned, a driving instrument is provided that can be used to disengage the snap-fit interface while securely retaining the screw. The driving instrument consists of a cannulated shaft with at least one prong or flat to engage the screw head, and a draw rod that inserts through the cannulated shaft to thread into the screw head.
To remove a screw, the device is held in place, either by other screws that have already been placed, or by holding the bone plate down by some other instrument. The screw is then rotated to disengage the thread from the bone. As the screw is rotated, the surgeon pulls the screw out of the snap-fit undercut to prevent thread stripping. Since the screw is securely gripped by the draw-rod driving instrument, the surgeon is able to apply sufficient tensile force to pull the head portion of the screw out of the undercut.
Another aspect of the present invention is that the bone plate of the invention is made from a composite material with radiolucent properties that allows for clear, undistorted postoperative MRI images to be produced. The composite bone plates do not contribute noise artifacts to the MRI image, and are also invisible to the imaging equipment. The bone screws of the present invention are made from titanium or a titanium alloy to act as clearly visible marker posts which provide the surgeon with postoperative position data.
In yet another aspect of the present invention, stress-controlling ridges are included on one surface of the bone plate to increase the fatigue life of the device. Without stress-controlling ridges present on the tensile surface of the plate, the highest stress concentrations due to the expected loading conditions occur in the vicinity of the undercut screw holes. This is a result of stress concentrations that can be approximated using fracture mechanics theories.
The stress-controlling ridges are located at a distance further from the neutral axis than the most extreme plane of the undercut screw holes. As a result, the bending stress on the plane containing the undercut screw holes is not the highest stress in the bone plate. Instead, the stress-controlling ridges are the highest stress regions of the bone plate. The ridges are produced so that they are continuous and unnotched, thus providing significantly improved fatigue performance compared to devices that do not have such ridges.
The present invention also provides an embodiment in which a fusion cage is incorporated into the bone plate. During deployment, the fusion cage, which is packed with a tissue graft, is placed in the area between two vertebrae. The bone plate is then affixed to the vertebrae in a manner consistent with other embodiments of the invention. Thus, this embodiment allows for increased bone graft and fusion to occur while retaining the aforementioned properties of the other embodiment of the present invention.
Still other advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description, accompanying drawings and appended claims.